Abstract
TiAl alloys are of interest to the aerospace and automotive industries (particularly for engine components) on account of their relatively low density and good mechanical properties at high temperatures. Processing routes involve melting and solidification of the alloy and require knowledge about the solidification morphology and microstructural texture evolution in the component. Among others, the Columnar-to-Equiaxed Transition (CET) of bcc β(Ti) dendrites is an issue of current interest. This article examines the results from solidification experiments where a combined Bridgman and power-down technique was implemented at four different cooling rates, using cylindrical samples of the TiAl alloy: Ti-45.5Al-4.7Nb-0.2C-0.2B (all at.%). Axial CET was observed in one of the samples and axial columnar to radial columnar microstructural transitions were observed in the others. A Bridgman Furnace Front Tracking Model (BFFTM), tailored specifically for use with the experiment apparatus, was used to estimate the transient thermal conditions and columnar growth conditions for CET and other microstructural transitions. An important link, due to the nature of the power-down technique, between the reversal of radial heat flow in the hot zone of the furnace and unwanted radial columnar growth, is explained using the model. Recommendations are made on how to avoid such growth, viz. use of low cooling rates and large sample diameters.

Key words
power-down technique, Bridgman furnace, gamma titanium aluminide, columnar to equiaxed transition, radial growth

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